The present invention relates generally to an exhaust system in a vehicle having a diesel engine.
Recent emissions regulations for vehicles employing diesel engines limit the amount of soot that the vehicles may emit. The soot is produced as a by-product of the combustion of the diesel fuel and is emitted with the vehicle exhaust. Diesel particulate filters (also called traps) added to the exhaust system limit the soot emissions sufficiently to meet the regulations.
Diesel particulate filters work by collecting the soot while allowing the exhaust gasses to pass through. As the vehicle operates, then, the soot builds up in the filter. This soot needs to be periodically eliminated from the filter in order to assure that the filter does not become clogged. A clogged filter can potentially cause damage to itself or the engine.
The particulate matter that builds up in the filter can be removed through a process called regeneration. Regeneration is performed by heating the diesel particulate filter to a temperature where the soot will burn away, thus cleaning out the filter. There are two general types of regeneration—active regeneration and passive regeneration. Passive regeneration occurs under vehicle operating conditions where the temperature in the diesel particulate filter will inherently rise sufficiently (from energy already in the exhaust gas stream) to burn away some of the soot. This is advantageous in that no extra energy input is needed for the regeneration process to occur. However, while passive regeneration is desirable, it cannot be counted on to sufficiently burn away the soot under all vehicle operating conditions. For example, passive regeneration may not occur at cold ambient temperatures or under certain driving conditions, such as extended idle or city driving.
Thus, active regeneration may be employed to assure that sufficient amounts of soot are burned away in the diesel particulate filter. Active regeneration may be accomplished by an electric or other type of heater located adjacent to or within the diesel particulate filter that is activated to raise the temperature sufficiently to burn the soot away. This type of regeneration requires extra hardware near the filter as well as an energy source and controls for operating such a heater.
Another type of active regeneration is where the exhaust stream is enriched with fuel by either late in-cylinder fuel injection or by direct injection of fuel into the exhaust stream. This fuel in the exhaust stream, as it burns, raises the temperature of the exhaust gas stream—and hence the diesel particulate filter—sufficiently to cause the regeneration process to occur. While this type of active regeneration eliminates the need for extra heater hardware near the filter, it does require extra energy input (extra fuel) to be used in order to accomplish the regeneration process. Moreover, this type of regeneration has the affect of raising tailpipe exhaust gas exit temperatures much higher than is typically experienced under normal operating conditions. These temperatures may possibly rise to a level that is environmentally undesirable.
Also, for vehicles with the added diesel particulate filter, the filter regeneration mode of engine operation may create noise in the exhaust that is different or louder than that produced during the normal mode of engine operation. Consequently, a conventional muffler may have reduced effectiveness in reducing unwanted noise emissions from the exhaust system during both the normal vehicle mode of operation and during the regeneration mode of operation.
It is desirable, therefore, to provide an exhaust system employing a diesel particulate filter with a means for adequately cleaning out the filter while minimizing the amount of additional energy input, minimizing the amount and cost of extra hardware needed to achieve the filter regeneration, minimizing the increase in tailpipe exhaust gas exit temperatures during regeneration, minimizing exhaust gas backpressure during a normal mode of operation, and reducing unwanted noise emissions from the exhaust system for both the normal and regeneration modes of operation.